Compositions obtained from the reaction of silicic acid esters such as tetraethyl orthosilicate, and organic tin compounds such as dibutyltin dilaurate and their use as crosslinking agents for organopolysiloxane compositions to form elastomers are described in German Patent No. 1,167,527 to Farbenfabriken Bayer Aktiengesellschaft. U.S. Pat. No. 3,186,963 to Lewis also discloses a catalyst system for vulcanizing silicone elastomer stocks at room temperature in which the catalyst is prepared by heating for at least 15 minutes at a temperature of from 80.degree. to 200.degree. C. a tin salt of a carboxylic acid and an alkylsilicate or partial hydrolysis products of said silicates.
U.S. Pat. No. 3,927,052 to Vizurraga discloses a polymerization catalyst for preparing polyesters which is obtained from the reaction of a first silicon compound of the formula ##STR1## with a second compound of the formula ##STR2## in a mole ratio of from 1.1 to 20 moles of the second compound per mole of the first compound for from two to six hours, in which M may represent a tin atom, R.sub.2 and R.sub.4 are chlorine or hydroxy groups and the other R substituents are selected from the group consisting of alkyl, acyl, alkoxy, aryloxy and hydrogen, provided that at least one of R.sub.5, R.sub.6, R.sub.7 and R.sub.8 is not hydrogen.
Another reference which discloses silicon-tin compounds as condensation catalysts in the preparation of organopolysiloxane elastomers in U.S. Pat. No. 4,137,249 to Wohlfarth et al. in which the silicon-tin compounds are prepared by reacting a silane of the formula R.sub.a Si(OR.sup.4).sub.4-a with a dialkyltin salt of the formula EQU R.sup.1 R.sup.2 Sn(OOCR.sup.3).sub.2
wherein R represents an alkyl or phenyl radical, R.sup.1, R.sup.2 and R.sup.3 are alkyl radicals, R.sup.4 is a monovalent hydrocarbon radical, a is 0 or 1 in a molar ratio of (4-a) moles of dialkyltin salt per mole of silane at a temperature of from 50.degree. to 160.degree. C., while removing from 80 to 100 percent of the theoretical amount of by-product ester formed from the reaction.
The products obtained from the reaction of silicic acid esters and an organic tin compound of this invention have certain advantages over the silicon-tin compounds described in the prior art in crosslinking diorganopolysiloxane-based compositions to form elastomers. For example, diorganopolysiloxane based compositions containing the product obtained from the reaction of a silicic acid ester and an organic tin compound can be crosslinked much more rapidly without decreasing the pot-life of the composition. Moreover, the crosslinking agents obtained from the reaction of silicic acid esters and an organic tin compound may be stored at room temperature or heated to a temperature up to 60.degree. C. without a substantial decrease in the crosslinking rate or without substantially altering the pot-life.
Therefore, it is one of the objects of this invention to provide a crosslinking agent for organopolysiloxane compositions. Another object of this invention is to provide a crosslinking agent for room temperature vulcanizable diorganopolysiloxane-based compositions. Still another object of this invention is to provide a crosslinking agent which is obtained from the reaction of a silicic acid ester and an organic tin compound. A further object of this invention is to provide a crosslinking agent which may be heated up to about 60.degree. C. without decreasing the rate of crosslinking or the pot-life.